This invention relates in general to vehicle anti-lock brake systems and in particular to a rear wheel anti-lock brake system which includes electronic brake proportioning.
Non-commercial vehicles are typically equipped with a hydraulic brake system having a master cylinder which is connected to wheel cylinders mounted adjacent to each of the vehicle wheels. The master cylinder is connected by a mechanical linkage to the vehicle brake pedal and the wheel cylinders are mechanically connected to the wheel brakes. When the vehicle operator depresses the brake pedal, the master cylinder applies hydraulic pressure to the wheel cylinders which, in turn, actuate the wheel brakes.
Typically, the vehicle brake system also includes a hydraulic proportioning valve connected between the master cylinder and the rear wheel cylinders to provide a balance between front and rear brake forces during a brake cycle to ensure adequate vehicle stability. Such brake force proportioning is designed so that, in the majority of braking cases, the front wheels will lock up before the rear wheels. This is because the locking of the rear wheels brings about a high risk of skidding while locking of the front wheels decreases steerability. Accordingly, the hydraulic proportioning valve controls the rear brake pressure with a fixed proportionality to the front brake pressure.
An anti-lock brake system (ABS) is often included as standard equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS, which controls all four vehicle wheels, includes a plurality of normally open and normally closed solenoid valves which are mounted within a control valve body and connected to the vehicle hydraulic brake system. A separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheel brakes during an ABS braking cycle. The pump is typically included within the control valve body while the pump motor is mounted upon the exterior of the control valve body.
It is also known to provide an ABS for the rear wheels only. Such a system is commonly referred to as a Rear Wheel Anti-Lock brake system (RWAL). Typically, RWAL does not include a motor driver pump, but utilizes the vehicle master brake cylinder as a source of pressurized brake fluid. While a RWAL has a limited volume of pressurized brake fluid available during an ABS brake cycle, elimination of the pump and pump motor simplifies the system and reduces the cost of the system.
Both four wheel ABS and RWAL include an electronic control module which has a microprocessor. The control module is electrically coupled to the pump motor, in a four wheel ABS, a plurality of solenoid coils associated with the solenoid valves and one or more wheel speed sensors for monitoring the speed and deceleration of the controlled wheels. A four wheel ABS typically includes a wheel speed sensor for both of the front vehicle wheels and a single wheel speed sensor for the rear wheels. Because only the rear wheels are controlled in a RWAL, only one wheel speed sensor is needed to monitor the rear wheel speed. This further reduces the cost of a RWAL in comparison to an ABS. The control module is typically mounted upon the control valve body to form a compact unit which is often referred to as an ABS electro-hydraulic control unit.
During vehicle operation, the microprocessor in the ABS control module continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to actuate the pump motor, in a four wheel ABS, and selectively operate the solenoid valves in the control unit to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
The brake proportioning function described above has been integrated into four wheel ABS, as described in U.S. Pat. No. 5,632,535, which issued on May 27, 1997. The brake proportioning described in the patent utilizes the wheel speeds measured by the front and rear wheel speed sensors to dynamically control the rear brake force with the solenoid valves included in the ABS for controlling the rear wheel brakes. The resulting electronic brake proportioning function is included in the four wheel ABS by a modification of the ABS control algorithm. Since the sensors and solenoid valves are already included in the ABS, electronic rear brake proportioning can be provided by the four wheel ABS without additional hardware. Accordingly, the hydraulic proportioning valve can be eliminated, which provides a cost saving.